Planar waveguide hygrometer and state sensor demonstrating supercooled water recognition

A sensor believed to be the first truly integrated optical sensor capable of detecting the liquid-solid phase transition of water is presented. The condensation, freezing, melting and evaporation of water are all detected with a planar silica Bragg grating operating in the 1.5 μm telecommunications window. Additionally, use of the sensor allows recognition of supercooled liquid at temperatures below the melting point of water. The device, well suited for integrated optics, is fabricated by direct UV writing with simultaneous definition of the grating. The Bragg grating is exposed and water is allowed to condense over it. Interaction with the evanescent field causes small changes in effective index (5 × 10−6) which can be detected, a sufficient sensitivity to identify the phase transitions of water clearly.